1. Field of the Invention
The present invention relates to an automobile body framework formed of a lightweight metal material, and also to lightweight metal framework members forming a front-end portion of the automobile framework.
2. Description of the Related Art
Aluminum is a metal that has high moldability and a strength equivalent to that of steel, yet is light in weight. Aluminum also has excellent impact energy absorption during deformation. Thus, in recent years, attention has been focused on using aluminum as the framework material of automobiles.
If the kinetic energy of an automobile framework at impact is balanced with an axial stress of an extrusion, then in order to generate a constant stress over an entire stroke, the form of the deformation needs to remain constant. That is, if during an impact, compressive deformation changes to bending deformation, the result is an abrupt change in stress. It is desirable to avoid such a situation.
In order to receive all axially imposed stress as compressive stress, a predetermined stable compression load needs to be generated in a impact absorption member. If the cross-sectional area of the impact absorption member is simply set, it is difficult to provide a stable deformation. Through experiment, the inventors have found that providing compressive deformation using, for example, a hollow, polygonal impact absorption member, results in a constant correlation between the length L of one side of the hollow material and its plate thickness t. However, if an attempt is made to set the bending rigidity of the framework based on this correlation, the length L becomes too large, lowering the automobile's space efficiency.
Further, although the lightweight metal material has a narrow elastic region and a high damping force caused by plastic deformation, it is similar to other metal materials in that the stress at the yield point is exceptionally high as compared with the average stress in the plastic region (about 1.5-2.0 times). For example, to absorb collision energy by compression deformation of an extrusion having a constant cross-section, the presence of a large peak at deceleration is not avoided with a conventional framework. That is, formerly, if the stress at the time of impact is lowered so that deceleration is endurable by a passenger, there is insufficient energy absorption due to lowered average stress. In contrast, if the damping force in the plastic region is raised, the peak deceleration becomes too large.
Moreover, once the aluminum material becomes plastically deformed, it is substantially impossible to restore the material to its original form. For example, if a portion of the material becomes deformed because of a small collision, an easy repair (e.g., by heating, striking, etc., as done with steel) cannot be made. Thus, a repair must be made on a large scale, and resulting in expensive repair costs.